Academic Dissertation to be presented with the assent of the Faculty of Medicine, University of Oulu, for public discussion in the Auditorium of the Department of Medical Biochemistry, on December 5th, 2001, at 1 p.m.

Abstract

Overlapping genomic clones coding for the α1 chain of mouse type XV collagen
(Col15a1) were isolated. The gene was found to be 110 kb in
length and to contain 40 exons. Analysis of the proximal 5'-flanking region
showed properties characteristic of a housekeeping gene promoter, and functional
analysis identified cis-acting elements for both positive and negative regulation
of Col15a1 gene expression. The general exon-intron pattern
of the mouse Col15a1 gene was found to be highly similar to
that of its human homologue, and comparison of 5'-flanking sequences indicated
four conserved domains. The genomic area encoding the end of the N-terminal
non-collagenous domain nevertheless showed marked divergence from the human form.
Due to the lack of two exons coding for the N-terminal collagenous domain and a
codon divergence in one exon, the mouse β1(XV) chain contains seven
collagenous domains whereas the human equivalent contains nine.

In order to understand the biological role of this protein, a null mutation in
the Col15a1 gene was introduced into the germ line of mice.
Despite the wide tissue distribution of type XV collagen, the null mice developed
and reproduced normally and were indistinguishable from their wild-type
littermates. After three months of age, however, microscopic analysis revealed
progressive histological changes characteristic of myopathic disorder, and
treadmill exercise resulted in greater skeletal muscle injury than in the
wild-type mice. Irrespective of potential anti-angiogenic properties of type XV
collagen-derived endostatin, the number of vessels appeared normal. Nevertheless,
ultrastructural analyses revealed markedly abnormal capillaries and endothelial
cell degeneration in the heart and skeletal muscle. Perfused hearts showed a
diminished inotropic response, and exercise resulted in cardiac injury, changes
that mimic early or mild heart disease. Thus type XV collagen appears to function
as a necessary structural component for stabilizing cells with surrounding
connective tissue in skeletal muscle cells and microvessels.

Mice lacking the type XV collagen homologue, type XVIII collagen, showed delayed
regression of blood vessels in the vitreous body of the eye and abnormal
outgrowth of the retinal vessels. This suggests that collagen XVIII plays a role
in regulating vascular development in the eye. Moreover, type XVIII collagen was
found to be important at the surface between the inner limiting membrane and the
collagen fibrils of the vitreous body. Col18a1 deficient
mice serve as an animal model for the recessively inherited Knobloch syndrome,
characterized by various eye abnormalities and occipital encephalocele. The
results presented in this thesis indicate diverse biological roles for the
closely related collagen types XV and XVIII.